The so-called tandem type color printer formed by arranging photosensitive drums in parallel corresponding to yellow, magenta, cyan and black respectively is known as an electrophotographic color printer.
The tandem type color printer includes developing rollers opposed to the photosensitive drums respectively. Electrostatic latent images are formed on the surfaces of the photosensitive drums. When the electrostatic latent images are opposed to the developing rollers following rotation of the photosensitive drums, toners are fed from the developing rollers to the electrostatic latent images. Thus, toner images are formed on the surfaces of the photosensitive drums respectively. The toner images of the corresponding colors are formed on the photosensitive drums respectively and overlappingly transferred to a sheet transported by a belt, whereby a color image is formed on the sheet.
A color printer having four photosensitive drums integrally attachable to and detachable from a main body casing is proposed as such a tandem type color printer.
The color printer according to the proposal includes drum subunits retaining the photosensitive drums respectively and a pair of side plates holding the drum subunits from both sides in the axial direction of the photosensitive drums. Developer cartridges retaining the developing rollers are detachably mounted between the side plates correspondingly to the drum subunits respectively. The drum subunits, the developer cartridges and the pair of side plates are detachably mounted on the main body casing as a drum unit.
Both end portions of drum shafts of the photosensitive drums protrude outward from both side surfaces of the drum subunits. Four shaft holes are formed in each side plate at regular intervals in the arrangement direction of the photosensitive drums. End portions of the drum shafts are inserted into the shaft holes respectively, so that the photosensitive drums are positioned.
Each side plate is made of a metal having a relatively low linear expansion coefficient. The side plate (hereinafter referred to as a “metal side plate” in this section) made of a metal has rigidity capable of keeping the relative positional relation between the photosensitive drums constant, while the same is heavier as compared with a side plate (hereinafter referred to as a “resin side plate” in this section) of the same shape made of resin. Further, the metal side plate disadvantageously requires a higher material cost than the resin side plate.
Therefore, each side plate may be formed by fixing a metal side plate and a resin side plate to each other so that the metal side plate retains the photosensitive drums and the resin side plate retains the developer cartridges and the like. According to the structure, the weight of the side plate and the material cost therefor can be reduced due to the partial employment of the resin side plate.
When the temperature around the side plate (the temperature in the color printer) changes, however, a difference in quantity of expansion/contraction is caused between the metal side plate and the resin side plate due to the difference between the linear expansion coefficients of the metal and the resin. When the difference in quantity of expansion/contraction is caused between the metal side plate and the resin side plate, the metal side plate may be deformed following expansion/contraction of the resin side plate, to result in a change (misregistration) of the relative positional relation between the photosensitive drums. Relative misregistration between the photosensitive drums leads to misregistration (color shift) between the toner images of the respective colors on the sheet.